1. Field of the Invention
The present invention relates to a compound semiconductor light emitting device such as a blue light emitting diode and a blue laser diode as well as its manufacturing method. This invention particularly relates to a light emitting device comprising a nitride-based compound semiconductor epitaxially grown on an insulative substrate such as a sapphire substrate, and its manufacturing method.
2. Description of Prior Art
Epitaxial growth of nitride-based compound semiconductors used for blue light emitting diodes, blue laser diodes and the like is generally performed on a sapphire (Al2O3) substrate whose lattice constant is similar to that of the nitride-based compound semiconductors. FIG. 23 illustrates an example of a basic structure of a blue light emitting semiconductor device comprised of a nitride-based compound semiconductor. A buffer layer 220 made of AlxGa1-xN (0≦X≦1) for example is formed on a sapphire substrate 210, and an n-type contact layer 230 made of n-type GaN doped with silicon (Si) for example is formed on the buffer layer 220. An n-type cladding layer 240 made of n-type AlxGa1-xN (0≦X≦1) doped with silicon (Si) for example is formed on the n-type contact layer 230. A multi-quantum well structure active layer 250 having a composition of AlaInbGa1-a-bN (0≦a, 0≦b, a+b≦1) for example is formed on the n-type cladding layer 240. On this active layer 250, a p-type cladding layer 260 made of p-type AlYGa1-YN (0≦Y≦1)doped with magnesium (Mg) for example is formed, and a p-type contact layer 270 made of p-type GaN doped with magnesium (Mg) is formed on the p-type cladding layer 260.
A p-type electrode 280 is provided on a surface of the p-type contact layer 270. A part of the laminated semiconductor layer is etched to expose the n-type contact layer 230, on which an n-type electrode 290 is provided.
Electric current does not pass between the electrodes when the electrodes are respectively provided on a rear surface of the substrate and a front surface of the semiconductor layer to bring a pair of electrodes like a conventional light emitting device comprising a conductive substrate because the sapphire substrate behaves as an insulator.
Therefore, as described above, a part of the semiconductor layer is removed from its front surface so that a semiconductor layer of one conductivity type is exposed and an electrode of the other conductivity type is formed on the remaining front surface. Thus, the nitride-based compound semiconductor light emitting device delivers performance by conducting electricity through a pair of electrodes, both of which are provided on the front surface side of the semiconductor layer.
With this structure, the light extraction efficiency is low since the pair of electrodes existing on one side of the semiconductor device shade a lot of light. Also the pair of electrodes existing on one side of the semiconductor device require wire-bonding at least twice. When a chip is mounted face down on a board, the electrodes on the chip must be accurately aligned with corresponding electrodes on the base, involving difficulty in precise alignment.
By the way a semiconductor light emitting device comprising a sapphire substrate with a contact hole to make contact with a semiconductor layer from the sapphire substrate side is disclosed in JP173235/1998, A. This semiconductor light emitting device comprises a sapphire substrate, wherein a rear side thereof is terraced, and a contact hole is provided to a thin-walled part of the terraced rear side of the substrate by reactive ion etching so as to make a semiconductor layer exposed.
It is surely possible to contact the semiconductor layer from the sapphire substrate side, and the electrodes are separately disposed on the substrate side and the semiconductor layer side in the semiconductor light emitting device disclosed in the above-mentioned specification.
However, the manufacturing process of this device becomes complicated because the substrate must be terraced in advance to form the contact hole by reactive ion etching, which may cause frequent cracks in the substrate.
One of the objectives of the present invention is to improve the light extraction efficiency. Another objective is to provide a chip which enables mounting of easy positioning with only once wire bonding, and leads to a reduction in the manhour. A further objective of the present invention is to provide a device with reduced numbers of manufacturing processes and increased process yield by reducing occurrence of cracks in substrates.